1. Field of the Invention
This invention relates to the promotion of low smoke emission and fire-resistance characteristics in a vulcanizable and halogen-free polyolefin mixture for application in compounds to isolate and cover cables and electric conductors.
2. Description of the State of the Art
One of the main applications of the fire-resistant and low smoke emission polymer compounds is as insulation for cables and electric conductors, where the main objective is to introduce better safety conditions in cable operation, above all in fire-hazard conditions, and where the physical properties and thermostability of the compositions should not deteriorate under operation conditions. The compositions are applied as insulations on cables and electric conductors in reduced thickness within a 10 to 15 mil range according to the norm for low tension primary cables, SAE J 1128, and for UL cables 125.degree. C. according to UL 44 SIS. The compositions present a good balance of the main properties such as chemical and electrical physico-mechanic processability with fire resistance, low toxicity and low smoke emission.
Thus, for example, in U.S. Pat. No. 5,256,488 cable insulating compositions are described that improve fire resistance and do not discolor or remove the copper conductor gloss after steam vulcanization. In this invention, these characteristics are obtained but using a mixture of different copolymers based on vinyl ethylene ester and alkyl ethylene acrylate with a low charge of different antioxidants such as pentaerythritol beta alkylthio propionate and steric phenol.
In U.S. Pat. No. 5,412,012, an insulating composition is also described, the main characteristic of which is to improve the adherence to the metal conductor and the composition of which is similar to the one of the previously mentioned patent, the only difference being the use of a mixture of antioxidant agents including thio diethylene bis(3,5-di-tert-butyl-4-hydroxyhydro cinnamate) compound.
In Mexican patent 162481 an insulating material made up of fire-retardant polyolefin based on an ethylene and vinyl ester of carboxylic acid copolymer and mixtures of acrylate is claimed, however this material includes halogenated materials and antimony trioxide, which are undesirable.
The main advantages obtained when developing said compositions are: (a) without the use of halogen based fire resistant materials to eliminate the potential risk of being in contact with hydrogen halide based smoke generated in a conflagration (b) without the use of carbon black charges it is possible to obtain compositions that can be colored (c) without the use of antimony trioxide the use of halogenated ingredients is avoided.
Hereinbelow this invention shall be described in a more detailed way but, of course, without limiting its scope.